36 9. Seismic retrofitting of reinforced concrete columns and frames using FRP Reinforced concrete columns Strengthening methods for reinforced concrete columns The earthquake resistance of many of the older RC columns is too low. Their flexural strength and ductility are not sufficient. On the one hand, these shortcomings are due to the fact that the lap splice lengths of the internal rebars are too short and, on the other, that the anchoring length of the internal rebars of adjacent structural elements is too short. Typical overlapping in the compression zone, with a length of 20 times the rebar diameter, is not sufficient to transmit all the post-elastic forces from the reinforcing rebars. Under seismic action insufficiently anchored rebars show an unacceptable behaviour. When exposed to cyclical bending moments and potential, additional, tensile loads, these struc- tures are subject to premature failure. This becomes especially critical in the case of foun- dation connections. If the projecting reinforcement exhibits too little overlapping, under seismic action it may form an undesirable plastic hinge. Unfortunately, this type of deficient construction detail is frequently found in older buildings and bridges. The transverse strength of RC structures is also often insufficient. One solution for remediation of these deficiencies, within supporting elements, is that of confinement. The earthquake resistance of a column with confinement reinforcement on its connections is improved due to the radial stresses. For the confinement (wrapping) of exi- sting structures various methods are available. Three different strengthening methods were compared in largescale tests: • Steel jackets • Concrete casing • FRP confinement The traditional methods with steel and concrete are well known and will therefore not be discussed. Reinforced concrete columns retrofitted with FRP jackets During recent years the performance of reinforced concrete columns retrofitted with FRP jackets has been verified in several scientific studies. Design concepts for the dimensio- ning of FRP confinements are available (e.g. M.J.N. Priestley, F. Seible, G.M. Calvi, "Seismic design and retrofit of bridges"). The confinement of columns with a rectangular cross section is more difficult than that of a column with a circular cross section. The radial stresses are concentrated at the corners. In order to achieve a defined confinement force (radial confinement), S&P has developed a prestressing system. Prestressing produces a defined three-dimensional stress level. In the case of non-prestressed FRP jackets, the three-dimensional stress level is only achie- ved when transverse strain of the concrete occurs. Prestressing increases the reinforcing 37 efficiency of the FRP wrap. With this new method sufficient radial stress can thus be pro- vided in the case of columns with a rectangular cross section. Unlike steel jackets, FRP wrap columns do not show any reduction in performance. Addi- tional tests showed that with FRP confinements potential shear failure can be avoided very effectively. Uni-directional confinement (horizontal) of columns with FRP jackets does not affect the vertical stiffness of the element, an essential part of seismic strengthening. Dimensioning of the FRP confinement is basically identical to the dimensioning adopted for steel stirrups. The limiting design strain of FRP is assumed as approx. 50% of the breaking elongation of the fibre. The high safety factor guarantees that the shearing mechanism of the reinforced concrete column is not reduced by excessive concrete strains. The FRP confinement acts in two ways: • Under tensile loading the effect of the FRP confinement on the plastic hinge region of the column is that the overlapping of the reinforcement, or of the anchoring of the longitudinal reinforcement respectively, is strengthened, and the post-elastic behaviour of the reinforcing bars can thus be fully utilised. • Under compressive loading of the plastic hinge region the confinement prevents buckling of the longitudinal reinforcement as well as spalling of the concrete cover. Confinement to improve flexural ductility The primary aim of the confinement is the enhancement of the flexural ductility. Columns with insufficient internal stirrup reinforcement cannot sustain large non-elastic rotations in the plastic hinge region. FRP confinements are suited to increase the flexural ductility of such supporting elements. Tests on circular columns retrofitted with FRP clearly indicated that they are able to increase the ductility more effectively than conventional steel jackets. The reason for this is the linear-elastic behaviour of the FRP confinement. Seismic respon- se can cause steel jackets to undergo tangential deformation. On unloading, residual pla- stic strains remain in the jackets. The effectiveness of the steel jacket is therefore reduced for each successive seismic response, and for each new load cycle higher hoop strains are required. The linear-elastic behaviour of FRP means that cumulative damage does not occur under cyclic loading. Successive cycles cause similar hoop strains. This improved behaviour, compared to steel jackets, is taken into account in the dimensioning concepts for FRP confinement that have been deduced from testing programmes. Circular columns The maximum concrete compressive strain εcu of the column can be calculated as follows: 2.5ρsfFRPεFRP εcu = 0.004 + [M.J.N. Priestley, F. Seible, G.M. Calvi] f’cc The ratio of volumes of the confinement ρS of a circular column is defined as follows: 4tFRP ρs = D fFRP and εFRP are the maximum stress and strain in the FRP confinement. 38 The concrete compressive strength f’cc of a confined reinforced concrete column is defined as follows: 7.94f’l 2f’l f’cc = f’c 2.254 1+ - -1.254 [M.J.N. Priestley, F. Seibel, G.M. Calvi] f’c f’c In the formula f’c represents the characteristic concrete compressive strength and f’l the effective lateral confinement stress (stress of the theoretical fibre cross section). From the expressions for εcu and ps, the required confinement strength tFRP (theoretical fibre thickness of the FRP confinement) can be deduced: 0.1(εcu - 0.004)Df’cc tFRP = fFRPεFRP Rectangular columns Push-pull tests on rectangular columns retrofitted with G-FRP likewise indicated an increase in ductility. Figure 43 shows a rectangular column retrofitted with G-FRP after failure. In the test a displacement ductility (failure) of µ∆ = 8 was reached. This corresponds to a displacement angle of approx. 4%. Figure 34: Rectangular column with GFRP confinement M.J.N. Priestley, F. Seible und G.M. Calvi, "Seismic design and retrofit of bridges" In several test series it could be shown that with rectangular cross sections com- pared to circular columns with an identical cross section an increase in ductility of approx. 50% is obtained. 39 The maximum concrete compressive strain εcu of the rectangular column is calculated as follows: 1.25ρsfFRPεFRP εcu = 0.004 + [M.J.N. Priestley, F. Seible, G.M. Calvi] f’ cc The ratio of volumes of the confinement ρs of a rectangular column is defined as follows: b+h ρs = 2tFRP bh In the formula b and h represent the dimensions of the cross section. From the expressions for εcu and ρS, the theoretical required FRP confinement thickness can likewise be deducted: 0.4(εcu - 0.004)f’cc bh tFRP = fFRPεFRP b+h Confinement area In Table 5 the required confinement areas (l) are indicated. Distinction is made between one-sided or double-sided restraint of the columns. La and Lb represent the distances from the support to the centre of moments. Restraint of the column Cross section of the column Column length Area One-sided D L D ≤ l ≤ 0.25 L Double-sided: Ma D La D ≤ l ≤ 0.25 La Mb Lb D ≤ l ≤ 0.25 Lb Table 5: Confinement area Confinement to improve the lap splices of the internal reinforcement The load transfer from the steel reinforcement into the concrete leads to the formation of micro-cracks in the concrete that will reduce the bond between steel and concrete. Retro- fitting with FRP jackets, and with prestressed FRP systems in particular, enhances this bond. 40 Dimensioning is carried out as follows: Abfs/µpls - fa ρs = 2 0.0015EFRP fa: active hoop stress Ab: surface of the longitudinal reinforcement fs: stress in the longitudinal reinforcement µ: coefficient of friction (1.4) p: area of influence of the fractured surface in the lap splice region ls: length of the lap splice The prestressed S&P A-Strap is specially suited for this field of application. Sheets with a low modulus of elasticity, such as sheets made of G fibres, are less suited for these appli- cations. Confinement area If strengthening of the lap splices is the only reason for the FRP confinement, a wrap above the lap splice is of no use. It only becomes necessary if the lap splice is not located in a plastic hinge region. Confinement for external shear strengthening FRP confinements, similarly to steel jackets, are very efficient in improving the shear resi- stance of RC-structures. Since FRP exhibits a linear-elastic behaviour until failure, the design value adopted for external steel reinforcement has to be slightly modified. A limiting design strain of the FRP of 0.2-0.3% has to be used. For this type of application the S&P C-Sheet 640 with a breaking elongation of 0.4% is ideally suited. Post-reinforcement of the shear resistance can be calculated as follows: Circular columns π VFRP = tFRPfFRPDcot θ [M.J.N. Priestley, F. Seible, G.M. Calvi] 2 D: cross section of the column θ: 35° Rectangular columns VFRP = 2tFRPfFRPh cot θ h: column dimension in parallel to the shear stress θ: 35˚ 41 Confinement area In the case of reduced shear resistance, the FRP confinement should be applied to the plastic hinge regions at a height of 2D for circular columns, or 2h for rectangular columns respectively. Flexural strength of the column As a result of the confinement of a column, its flexural stiffness is increased and it is thus subject to higher forces. This is a critical issue and needs careful attention. The increase in flexural strength depends on the elected strengthening method, the material parameters and the shape of the columns. Column Reinforcing Steel Concrete FRP Circular Plastic hinge 10 - 20 20 - 50 0- 5 Shear 20 - 40 25 - 75 0- 5 Rectangular Plastic hinge 20 - 40 20 - 50 0 - 10 Shear 40 - 70 25 - 75 0- 5 Table 6: Increase in flexural strength (%) FRP retrofitting causes a substantially lower increase in flexural strength compared to the application of steel jackets or concrete casing. Comparison of strengthening methods A comparison of the different strengthening methods is given in the following table. The results demonstrate the favourable behaviour of FRP confinements compared to conven- tional procedures. The main benefit of the FRP confinement is the minimal increase in flexural strength, despite its high capacity in improving ductility. As a result, the FRP retro- fitted structural element is not subject to additional forces, and premature failure of the framework is thus prevented. A defined three-dimensional stress condition is obtained using the prestressed S&P A-Strap. Prestressing provides an increase in concrete compressive strength, without producing large axial compressive strains in the load bearing element. The increased com- pressive strength allows higher loads to be supported. The application of FRP confinements is fast and easy. Downtimes are therefore substan- tially reduced. Furthermore, FRP jackets are thin and require less adaptation of adjacent building components. 42 Radial stresses Strengthening method Increase in Application Dimension Corrosion in flexural reduction Increase strength weight Steel jackets - - - - -- -- Concrete casing - - - -- -- 0 FRP confinement + + ++* + + ++ Table 7: Comparison of reinforcing methods * Prestressing without problems Reinforced concrete frames For a correct design of seismically endangered structures the structural engineer must be aware of the typical types of damage caused by earthquakes. The most frequent failure modes of reinforced concrete frames are described below. “Short Column” Insufficient, transverse, load-bearing capacity in the beam/column or beam/slab connections can cause shear cracking in the concrete at an early stage of loading. This crack formation causes the internal stirrup reinforcement to open and thus leads to failure of the element. The appli- cation of FRP jackets to improve the transverse load bea- ring capacity on the potential plastic hinge regions of the column can prevent this failure mode. Since the overall ductility of the structure must be taken into account also, the woven S&P G-Sheet 90/10 or the prestressed S&P A-Strap are specially suited for this type of application. FRP confinement with high modulus C fibres is less suited for this application as the requirements regarding the over- all ductility of the structure would be fulfilled to a lesser degree. Figure 35: "Short Column" 43 Weak junction points Junction points that are too weak or those with insufficient load capacity because of reduced cross sections can be sub- ject to other failure modes. Insufficient lap splice lengths of the longitudinal reinforcement at the ends of the columns lead to an additional frequent failure mode. Strengthening can be achieved by CFK laminates applied into longitudinal slots or to the surface, with additional wrapping of S&P G- Sheet 90/10 or the prestressed S&P A-Strap over the junc- tion points. Figure 36: Weak junction point Plastic hinge regions Beams subject to flexure with insufficient shear reinforcement can exhibit failure in the plastic hinge regions, as shown in the Figure 37. Retrofitting is carried out using the high modulus S&P C-Sheet 640 as confinement reinforcement. A further failure mode is the insufficient flexural strength of the beam at mid-span, or near the supports. In this case S&P Laminates CFK are applied into slots or onto the surface. Figure 37: Shear failure in a plastic hinge region Methodology of retrofitting with FRP In numerous research studies it has been verified that retrofitting of reinforced concrete in the plastic hinge regions provides enhanced load capacity and thus improves the ductility of the reinforced concrete frame. The effectiveness of the S&P G-Sheet or S&P A-Sheet in enhancing the ductility has been verified in push-pull tests. The tests further show that GFRP and AFRP provides a higher increase in ductility than a C fibre jacket. This is a benefit of the higher, ultimate strain of the glass fibre. Ideally suited for seismic retrofitting of columns are systems that cause either an increase in hoop stresses on the plastic hinge regions or over the entire length of the column. 44 Figure 38: Retrofitting of a reinforced concrete frame Large-scale tests indicate that G- or A-FRP confinements provide better technical benefits and are more economical than steel jackets. In the case of FRP confinement of the entire column or at its ends, concrete failure occurs at larger strains. The reduction of transverse strains provided by the FRP confinement also helps to minimise buckling of the longitudinal reinforcement. Prior to the confinement with FRP, structural repair of cracks in the substrate should be carried out using epoxy resin injection. 10. Explosion and impact protection using S&P FRP systems Explosion protection Structures can br frequently damaged by exposure to explosions or the effects of bombs. Protection against explosion can also be a requirement of the chemical industry. Whilst explosions in industrial buildings can be estimated and the necessary protection thus be designed, estimating the effects of a bomb is impossible. Traditional industrial buildings are often insufficiently reinforced. Masonry structures with little reinforcement are likewise seen in practice. Such structures offer only a minimal resistance to explosion hazards. Conven- tional strengthening methods using steel are costly and their application is time-consu- ming. FRP provides a time saving and economical solution.